Prior to the 1990s, power line communication devices were either built entirely with analog signal processing circuitry (e.g., power line based intercoms and baby monitors) or they were built with analog filtering followed by digital detection circuitry (e.g., X-10 devices).
In the early 1990s, the first power line communication devices to incorporate digital signal processing (DSP) for filtering and related receive functions were introduced. The architecture of these early DSP based devices is illustrated in FIG. 1. Typical DSP based power line transceivers use the same basic architecture as illustrated in FIG. 1.
FIG. 1 illustrates high-pass filters 110 and low-pass filters 115, which in some cases are implemented as a band-pass filter, followed by an automatic gain control (AGC) amplifier 120. The purpose of the initial high-pass filtering is to effectively remove any AC mains voltage (e.g., 50 or 60 Hz power frequencies), while also attenuating low-frequency mains noise that is below the communication band. The purpose of the low-pass filtering is to prevent aliasing products from later analog-to-digital conversion (ADC).
The initial filtering is followed by the functionality of the AGC amplifier 120. The AGC amplifier 120 selects the highest available gain to provide high sensitivity to reception of weak signals when there is little noise after processing by the initial high and low-pass filters. The AGC amplifier 120 also avoids saturation of the ADC when significant noise remains, after initial filtering, by selecting a lower gain setting in that instance.
The detector 125 determines when the AGC gain can be maximized without overload and sets it accordingly. The detector 125 also determines when residual noise levels are such that a lower gain setting must be selected in order to avoid overload, and thus selects a lower gain/lower sensitivity setting. The output of the AGC amplifier 120 is then fed to the ADC 130. The digital output of the ADC 130 is then fed to a DSP block 135 for higher selectivity filtering, and other receiver processing.
A table showing the AGC gain range and ADC equivalent number of bits (ENOB) for several current state-of-the-art power line communication devices is shown in Table 1. As used herein, ENOB is defined to be (SNR−1.76)/6.02, where SNR is the ratio of signal power to noise power expressed in decibels. When there is very little noise on the channel and the AGC is set to a higher gain value, these devices can receive data packets that have been attenuated by 65 to 90 dB relative to a full scale transmitted signal prior to channel attenuation.
TABLE 1Supplier ASupplier BSupplier CAGC gain range (dB)623018ADC ENOB (bits)~10~10~11Sensitivity (on quiet~80~65~80channel in dBFS)
A limitation of the architecture of FIG. 1 is manifest when large amplitude signals exist on the power mains that are adjacent to the communication band (this is sometimes referred to as cross-band interference). In this case these large amplitude signals cause the AGC gain to be set to a lower value, reducing the sensitivity of the receiver to recovery of intended receive signals that have been attenuated by the communication channel. In this case the receiver must use of a lower gain setting, thereby impairing its ability to recover attenuated messages. The described limitation is illustrated with the frequency domain plots of FIG. 2. As seen in the frequency domain plot 200, the attenuated incoming packet 210 is accompanied by a strong “other” signal 215. This strong “other” signal causes the AGC gain of the AGC amplifier 120 to be set low. As a result, and as seen in the frequency domain plot 205, the intended packet 210 is lost below the ADC noise floor 220.
The case illustrated in FIG. 2 is becoming increasingly common. Regulatory norms in European Committee for Electrotechnical Standardization (CENELEC) countries divide the power line spectrum below 148.5 kHz into multiple bands. CENELEC standard EN 50065-1 references “A-band, B-band, C-band and D-band to designate the frequency bands 3 kHz to 95 kHz, 95 kHz to 125 kHz, 125 kHz to 140 kHz and 140 kHz to 148.5 kHz respectively”. In CENELEC countries, EN50065-1 specifies that the A-band “ . . . shall only be used for applications for monitoring or controlling the low-voltage distribution network . . . ” while the B, C and D-bands are available for applications either within homes, commercial or industrial premises. In addition, EN50065-1 specifies that the B through D bands may also be used for, “Control and monitoring equipment installed on or connected to the low-voltage distribution network external to premises.”
When the spectrum is divided such that multiple users attempt simultaneous communication on the same distribution wiring in adjacent frequency bands, this results in situations where the above described limitation occurs. One example of such a situation is when an electric utility utilizes the A-band to communicate between each electricity meter and a data concentrating device located near the associated mains distribution transformer, while the C-band is used for communication from, or near, the electricity meter to other locations within the serviced home or commercial establishment. It is not uncommon for messages sent by a data concentrating device to be attenuated by 60 to 80 dB by the time they arrive at an electricity meter. At the same time a C-band device that is located very near the electricity meter may be transmitting a full strength signal directly adjacent to the electricity meter. With prior art architectures such as illustrated in FIG. 1 the electricity meter would be unable to recover the attenuated A-band message from the data concentrator because the nearby C-band signal is forcing the A-band receiver to a low-gain/low-sensitivity state. This is precisely the case illustrated in FIG. 2.
Another, example where the prior art architecture of FIG. 1 becomes a limiting factor is when street lighting devices that employ C-band communication share the same distribution wiring as A-band Smart Grid devices. When an A-band and C-band device are located adjacent to each other the presence of a strong transmission signal from one results in the other having reduced sensitivity and missing attenuated messages due to the AGC amplifier entering a low gain state. Note that even though the target receiver may have higher selectivity filtering inside the DSP unit, that does not resolve the problem since the AGC amplifier gain must still be lowered to avoid distortion that would spread the C-band signal into the A-band making it unrecoverable within the DSP. Note that it is generally not practical or economical to provide very high selectivity filtering ahead of the AGC amplifier as a means to improve adjacent channel selectivity.
It is not only adjacent band intentional communications that can cause an AGC amplifier to select a low-gain/low-sensitivity setting. Certain devices that are connected to the AC mains have emissions large enough to cause the same effect.
The above described limitation of prior art power line communication devices is not limited to those operating in CENELEC countries. In the United States, power line communication is permitted below 535 kHz and Federal Communications Commission (FCC) regulations do not impose any division of the mains communication spectrum into different bands. Most existing and emerging international industry standards have elected to adhere to the same CENELEC band structure, except in some instances allowing of another band from approximately 150 kHz to approximately 500 kHz. Examples of these standards include:                ITU-T G.9902 Narrow-band orthogonal frequency division multiplexing power line communication transceivers for ITU-T G.hnem networks        ITU-T G.9903 Narrow-band orthogonal frequency division multiplexing power line communication transceivers for G3-PLC networks        ITU-T G.9904 Narrow-band orthogonal frequency division multiplexing power line communication transceivers for PRIME networks        ISO/IEC 14908-3 Information technology—Control network protocol Part 3: Power line channel specification        P1901.2/D0.07.00 Draft Standard for Low Frequency (less than 500 kHz) Narrow Band Power Line Communications for Smart Grid Applications        
As more power line communication devices are deployed in conformance to the above standards the problem of a device transmitting in one band causing a nearby device to be unable to receive an attenuated signal in an adjacent band will increase.
High performance audio ADC technology typically operates with sample rates between 44.1 and 192 k samples per second with 16 to 20 ENOB in a 20 kHz bandwidth. High performance audio ADC technology commonly employs a form of sigma-delta modulator (SDM) (also known as Delta-Sigma Modulators or over-sampled analog to digital converters) which converts its input signal to a very small number of bits (e.g., 1 to 5 bit width stream) at a multi-megahertz sample rate. The small bit-width of the modulator results in high noise levels, but the noise is spread across a band that is much wider than the audio band of 20 Hz to 20 kHz.
Audio SDMs further shape this quantization noise spectra to minimize it in the band below 20 kHz, while allowing it to grow above the audio band. As a result, the noise density increases above the audio band where the noise is less harmful. This is accomplished with a modulator loop gain that remains high to 20 kHz and diminishes above the audio band (high loop gain results in quantization noise being minimized while frequencies with lower loop gain allow the noise error to increase). A digital filter and decimator are then employed in such a way to filter out quantization noise (as well as other signals) above the audio band.